Automatic injection devices offer an alternative to manually-operated syringes for delivering therapeutic agents into patients' bodies and allowing patients to self-administer injections. Automatic injection devices have been used to deliver medications under emergency conditions, for example, to administer epinephrine to counteract the effects of a severe allergic reaction. Automatic injection devices have also been described for use in administering anti-arrhythmic medications and selective thrombolytic agents during a heart attack (See, e.g., U.S. Pat. Nos. 3,910,260; 4,004,577; 4,689,042; 4,755,169; and 4,795,433). Various types of automatic injection devices are also described in, for example, U.S. Pat. Nos. 3,941,130; 4,261,358; 5,085,642; 5,092,843; 5,102,393; 5,267,963; 6,149,626; 6,270,479; and 6,371,939; and International Patent Publication No. WO/2008/005315.
Conventionally, an automatic injection device houses a syringe and, when operated, causes the syringe to move forwardly and a needle to project from the housing so that a therapeutic agent contained in the syringe is ejected into a patient's body. An automatic injection device typically includes a plunger with a distal end that is seated on a firing body before firing. In order to fire the device, a patient depresses a firing button which disengages the distal end of the plunger from the firing body and that allows the plunger to move the syringe forwardly.
Certain conventional devices may fail to fire or may fire after a delay if the firing button is not depressed with sufficient force, for example in the case of a rheumatic patient who is unable to exert high forces on the firing button. Even when a sufficient force is applied to the firing button, certain conventional devices may fail to fire or may fire after a delay due to conventional configurations of one or more structural components of the devices. These problematic firing patterns or misfires, i.e., failing to fire or firing with a delay, may be referred to as delayed delivery of an injection.